This invention relates to novel random graft copolymers in which homopolymers of .alpha.-substituted-.beta.-propiolactones and/or copolymers thereof are randomly grafted to base polymers which are amorphous to x-rays at a temperature of 130.degree. C. or below.
The random graft copolymers described herein are formed by contacting an amorphous base polymer having, on the average, at least one random anionic site thereon,
WITH UP TO 150% BY WEIGHT, BASED ON THE BASE POLYMER, OF AT LEAST ONE .alpha.-SUBSTITUTED .beta.-PROPIOLACTONE IN AN AMOUNT SUCH THAT THE RATIO OF THE MOLES OF .beta.-LACTONE TO THE MOLES OF ANIONIC SITES ON THE BASE POLYMER VARIES BETWEEN ABOUT 3 TO 1000, AND
POLYMERIZING SAID .beta.-LACTONE TO FORM CRYSTALLIZABLE POLYMER SIDECHAINS, SAID SIDECHAINS HAVING A LENGTH OF BETWEEN ABOUT 3 TO 1000 MONOMER UNITS AND A WEIGHT OF UP TO ABOUT 60 PERCENT OF THE GRAFT COMPOSITION,
SAID BASE POLYMER CHARACTERIZED IN THAT IT IS AMORPHOUS TO X-RAYS AT A TEMPERATURE OF 130.degree. C. or below, and has a molecular weight of above about 2,000.
As employed herein, the expression "random graft copolymer" refers to a composition consisting essentially of base polymer having an average of one or more polylactone sidechains randomly grafted thereto, there being no particular order in the location on the base polymer molecule of said grafted sidechains. Whereas conventional block copolymers of the simple ABA and ABC type have A and/or C segments characteristically attached at the ends of the backbone chain, B, the novel random graft copolymers show no such characteristic order or pattern. In fact, any amounts of such simple ABA and/or ABC type copolymer which might be present along with the novel random graft copolymer would be statistically negligible. However, the random graft copolymer can have a portion of its polylactone content (other than in at least one randomly located sidechain) attached at either or both ends of the backbone chain, since the amorphous base polymer chain can have anionic graftsites at either or both ends.
The term polymer(s) used herein refers to organic homopolymer(s) and/or copolymer(s), including terpolymer(s), etc., depending upon the context in which the term is employed.
The term "consisting essentially of" as used herein means that materials so described may include unspecified minor ingredients which do not materially affect the basic and novel characteristics of the invention. In other words, this term excludes unspecified ingredients in amounts which prevent the novel characteristics of this invention from being realized
The expression "base polymer" as employed herein encompasses polymers that contain at least one randomly located anionic site thereon. If a polymer contemplated to be used as a base polymer does not have the requisite random anionic site(s), then, said polymer must be susceptible to treatment as disclosed in the section, "Process for Making the Graft Compositions," in order to have said random site(s) formed thereon.
The base polymers employed herein can be solids or viscous liquids at ambient temperatures. The "amorphous" base polymers useful herein are those that are normally amorphous to x-rays at a temperature of 130.degree. C. or at some temperature below 130.degree. C. The expression "amorphous" also includes the characteristic that said base polymers soften or melt at 130.degree. C. or below. The base polymers will show amorphous type patterns in x-ray diffractograms at and above the lowest temperature at which said polymer contains amorphous segments.
The average molecular weights of the base polymers are from about 2,000 to about 10,000,000. The lower limit is a number average molecular weight as measured by vapor phase osmometry. The upper limit is a weight average molecular weight as measured by light scattering or viscosity. Regarding light scattering measurements, see, for example, "Physical Methods of Organic Chemistry" Part 3, Chapter XXXII, pages 2107-2145, Interscience Publications, Inc., N.Y. (1960).
It should be understood that the numbers employed herein to describe the novel random graft copolymers represent average values. The normal distribution of molecular weights, comonomers, functionality, etc., in a polymer composed as it is of long chain molecules of varying lengths makes such usage necessary and conventional. For instance, numbers given for the following graft copolymer characteristics represent averages of values: molecular weights of base polymers, numbers of sidechains per base polymer molecule, and degree of polymerization (DP) of the crystallizable sidechains.
Pivalolactone as used herein is the common name for the lactone of 2,2-dimethyl-3-hydroxypropanoic acid. The term crystalline or crystallizable as applied to the contemplated polylactones means that they can develop crystalline domains with or without drawing and/or annealing to form crystalline polymer. One method for detecting crystallinity is by examination of thin sections of the polymer through crossed polarizing filters under a microscope. Under such examination, crystallinity can usually be observed by the appearance of spherulites. Alternatively, crystallinity can be detected by differential scanning calorimetry, wherein the appearance of a melting endotherm is taken as an indication of the presence of crystalline structure. Extremely small crystalline domains are detectable by transmission electron microscopy using appropriate staining techniques. For a discussion of differential scanning calorimetry, see "Organic Analysis", Vol. IV, pp. 361-393, Interscience Publishers Inc. (1960).
The desirable sidechain length will depend in part on the degree of crystallinity of the polymer or copolymer of the particular .beta.-lactone or .beta.-lactones being used. Pivalolactone polymerizes to polymers that are highly crystalline. Other operable .beta.-lactones give polymers with relatively lower degrees of crystallinity. In general, the lower the degree of crystallinity of a side chain, the higher the DP thereof that will be desired to realize a given physical characteristic.
The novel graft copolymers taught herein have unusually good and surprising properties which may be attributed to the strong tendency of grafted polypivalolactone segments to crystallize one with another. As a result of this strong tendency, base polymer molecules which bear the grafted polyester are, in effect, bound together at the point where polyester segments crystallize. Where more than two polyester graft segments exist per chain, there is therefore possible the reversible formation of a three-dimensional network, much as in conventional (irreversible) vulcanization of natural rubber and synthetic elastomers. As a result, the graft polymers of this invention have unusually good resistance to creep and compression set, which may be defined as resistance to permanent deformation.
In addition, certain of the compositions of this invention, without curing, exhibit high elastic recovery, high tensile strength at break and high elongation at break, fully equivalent to those of conventionally vulcanized (cured) rubbery materials. Moreover, certain of these compositions show high tensile strength and modulus values unattainable by conventional curing of the corresponding base polymers by standard techniques without the addition of reinforcing fillers such as activated carbon black. The crystallized graft segments provide "internal reinforcement" and thus it may be unnecessary to undergo expensive bulk mixing of a foreign reinforcing agent, such as carbon black, to achieve high tensile strength and modulus properties.
The novel copolymers of this invention are strong form-stable materials at ordinary service temperature; they are viscous liquids above the melting points of the grafted polyester segments and can be processed as thermoplastic materials Thus, they can be formed into useful articles by conventional techniques such as compression and injection molding and the like at temperatures above the graft polymer melting point. On cooling, new network formation occurs by virtue of the crystallization of graft chains, as previously discussed, and the composition takes the shape of the mold cavity. This change from solid to liquid and back can be repeated several times without damage to the copolymer, making possible reuse of mold flashings and reject articles and the like. Short time cycles can be employed as opposed to more lengthy periods often required for conventional curing.
The graft copolymers of this invention also exhibit improved resistance to high temperature deformations when compared to the base polymer. The deformation temperatures of the graft copolymers are higher than the deformation temperatures of the corresponding base polymers. Thus, the temperature range of practical use of the base polymer is extended. Also, service life at a given temperature is improved because of the pervasive resistance to creep and to compression set. Solvent resistances of the novel graft copolymers are markedly better than those of the corresponding base polymers.
The chemical and physical properties of the graft copolymers can be varied over a wide spectrum by careful selection of the base polymer. For example, base polymers containing halogen or polar substituents give products with high resistance to hydrocarbon oils. Saturated hydrocarbon base polymers such as those obtained by copolymerization of simple olefins by coordination catalysts (ethylene-propylene-butylene, etc.) can be expected to provide high resistance to ozone and oxygen attack. Base polymer structures similar to natural rubber, e.g., 1,4-cis-polyisoprene, provide highly resilient copolymer products.